Conventionally, known types of steel materials for an underground continuous wall compositely integrated by combining steel sheet pile and H-beams, used when constructing a retaining wall or underground continuous wall (composite sheet pile) include (1) steel materials for an underground continuous wall in which a linear-shaped steel sheet pile or a wall-shaped steel sheet pile is secured to a flange of the H-beam by welding (for example refer to patent document 1).
Furthermore, in a known type of steel material for an underground continuous wall which is more rigid than the steel material for an underground continuous wall of (1) above, a U-shaped steel sheet pile is combined with H-beams, I-steel sections, or T-steel sections (for example refer to patent documents 3 through 6).
When constructing an underground continuous wall, because the underground continuous wall is constructed by linking a large number of the above-described steel sheet piles together in the horizontal direction, a large cost is incurred. If the length dimension of individual steel sheet piles in the steel material for an underground continuous wall can be reduced, in an underground continuous wall or retaining wall constructed using a large number of steel sheet piles, the weight of the steel material for the underground continuous wall is reduced, and accordingly transportation is easier. Furthermore, installation is also simplified, enabling the duration of construction work to be reduced. Consequently, an underground continuous wall can be constructed inexpensively, and construction costs can be reduced, providing a significant effect. For this reason, a lightweight and inexpensive steel material for an underground continuous wall is desired.
In the case where the H-beam is secured into a channel of a U-shaped steel sheet pile, because the H-beam is secured to the inside surface of the U-shaped steel sheet pile where the flanges are inclined at a greater angle, when the wall is driven into the ground, a problem occurs whereby the earth inside the channel in the U-shaped steel sheet pile tends to become compacted, causing the channel to become blocked. In contrast, the combination of a hat type steel sheet pile and an H-beam forms a gentler angle of incline than the U-shaped steel sheet pile, providing an advantage whereby the compaction of earth is reduced, alleviating the problem described above.
However, with a hat type steel sheet pile 2 manufactured by rolling such as that shown in FIG. 10, if its dimensions are changed, large equipment costs are incurred at the rolling facility. Existing hat type steel sheet piles include the hat type steel sheet piles 2 of the dimensions (in mm) shown in FIG. 10 and FIG. 11. In these hat type steel sheet piles 2, flanges 5 inclined so as to spread outward, are connected integrally to each end of a web 7, and arm sections 3 and 4 running parallel to the web 7 are connected integrally to the respective flanges 5, and at the ends of the arm sections 3 and 4, joints 14 (14a and 14b) are integrally formed, giving a hat shape in cross-section. The left and right joints 14a and 14b are point-symmetric with respect to the center point of the central axis of the arm sections 3 and 4, such that when the joints 14a and 14b of adjacent hat type steel sheet piles 2 are fitted together, the hat type steel sheet piles 2 can be arranged upon the central axis of the arms.
An advantage of the hat type steel sheet pile 2 is that because the hat type steel sheet pile 2 includes the inclined flanges 5 and the arm sections 3 and 4 at both ends thereof, the wide width dimension of the sheet pile means that fewer sheet piles need to be driven, allowing an inexpensive wall to be constructed. On the other hand, there has been a problem in that a hat type steel sheet pile with high flexural rigidity cannot be manufactured inexpensively and easily without changing the width dimension of the sheet pile.    Patent document 1: Japanese Unexamined Patent Application, First Publication No. S62-133209    Patent document 2: Japanese Unexamined Patent Application, First Publication No. H11-140864    Patent document 3: Japanese Unexamined Patent Application, First Publication No. S55-68918    Patent document 4: Japanese Unexamined Patent Application, First Publication No. H06-280251    Patent document 5: Japanese Unexamined Patent Application, First Publication No. 2005-127033    Patent document 6: Japanese Patent No. 3603793